Where some humans like us see a warm friendly face that we want to love and be kind to.

Other humans feel threatened unconsciously and have a strong desire to kill the animal.

Others have very little empathy and to them killing is fun like a bit of excitement no different than knocking on someones door and running away.

The worst kind of human gets pleasure from the animals pain, or even sexual pleasure, this trait evolved in humans because we partly evolved into a predatory animal (canine teeth meat eating) and so its important for predatory animals including humans to enjoy killing and causing an animal distress, it makes sense in evolution to enjoy and get rewarded for it since it promotes survival for the meat eating animal.

I hope so much that one day we can remove through eugenics all the sadists and low empathy people from the human gene pool, because these damaging instincts are no longer needed and oh so vile/evil/destructive in terms of the suffering they cause.

Thats partly WHY people like this do it, other factors like abuse to children can make what would have been a pleasant human genetically, into a low empathy or sadistic human, and the animals or other people pay the price for it. This is another evolutionary adaption/mechanism.